In this article we will discuss about the classification of oedogoniales.
Family Oedogoniaceae:
This family includes chiefly the fresh-water forms which prefer to grow in quieter situations and are often attached to submerged plant parts or any other object. The plant body may be of branched or un-branched filament. The distinguishing feature of the family is the presence of a large number of flagella at the anterior ends of the motile reproductive structures—zoospores, androspores, and the male gametes.
Sexual reproduction may be heterogamous or even oogamous.
Genus Oedogonium:
It is a common fresh-water alga which grows in quiet water, usually attached to other plant parts and is abundant in ponds and ditches. Free-floating condition is not also very rare. Its occurrence in running water is rather less common.
The outstanding characteristic features of this genus are, cell division resulting in cap formation, swarmer’s of varied size difference and function with a crown of flagella at the anterior end, oogamy with special dwarf male filament taking part in the sexual process, and clear sex segregation in the post-fertilization stages.
The plant body consists of filamentous un-branched thallus of cylindrical cells arranged end to end. The mature filament is differentiated into base and apex and consists of vegetative cells, antheridia, and one or more oogonia (Fig- 63A, C&D).
The basal cell flattens into a disc-like structure forming a holdfast (Fig. 63A), with which the filament attaches itself with the substratum, whereas, the apical cell is rounded. Each of the vegetative cells contains a single reticulate chloroplast with pyrenoids (Fig. 63B).
Each cell contains single large nucleus which lies near the middle of the cell remaining embedded in the cytoplasm just within the chloroplast. Certain cell in every filament has one or more ring-like markings which are known as caps (Fig. 63A & B). Oedogonium has no apical growing point. Cell division may take place in any cell of the filament except the basal cell.
The mode of cell division resulting in the production of a cap is rather unique of its kind.
With the migration of the nucleus towards the distal end of the cell, the cell division starts. The nucleus then divides mitotically, and simultaneously, there appears a ring of wall material (thickening ring) which encircles the inner face of the lateral wall just below the distal end of the ceil (Fig. 63E to G).
The division of the nucleus is followed by the appearance of a septum in between the two daughter nuclei (Fig. 63H). The septum remains floating for some time without being connected with the lateral wall (Fig. 63H-J).
The protoplast soon elongates as a result of which the newly formed ring on the lateral wall is stretched resulting in the breaking of the lateral wall in the traverse direction at the level of the ring. The ring becomes stretched further to form a new wall (Fig. 63I & J), although a little of the original wall remains at the upper end of the cell and is known as cap.
The elongation of the protoplast also causes the displacement of the floating septum upward until it takes up a permanent position near the lower end of the new wall developed from the stretched ring (Fig. 63K). Often, an individual cell may undergo several successive divisions, each leaving a characteristic cap, the number of which will denote the number of divisions the cell has undergone (Fig. 63B).
In addition to fragmentation of the filaments, reproduction may also be by the asexual and sexual methods. In the asexual method large multi-flagellate zoospores are produced singly in the vegetative cells which are then transformed into zoosporangia. In this event, the whole content of the cell gradually withdraws itself from the cell wall contracting to form a pear-shaped body (Fig. 64A).
At the same time a clear, colourless spot appears on one side of the contracted protoplasmic body around which is developed a characteristic ring of flagella (Fig. 64B). Ultimately a multi-flagellate zoospore with a ring of flagella arranged around colourless spot in the anterior region, is developed.
The zoospore is liberated being enclosed in a mucilage sheath, by the breaking across of the cell which has functioned as a sporangium (Fig. 64C & D).
A zoospore is almost spherical or pear-shaped and deep-green, but there is a well-marked colourless beak around the base of which short flagella arise (Fig. 64E). Such a type of zoospore is designated as stephanokontan zoospore. Eventually the zoospore comes to rest, attaches itself by its colourless end, withdraws its flagella, forms a cell wall, and divides to form a new filament (Fig. 64F & G).
Sexual reproduction is oogamous, with the production of large non-motile egg and small antherozoids. Some species are monoecious or homothallic (oogonia and antheridia develop on the same filament), others are dioecious or heterothallic (oogania and antheridia on different filaments).
Depending on structure and distribution of antheridia, species of Oedogonium may be grouped’ as: macrandrous type, and nannandrous type.
In macrandrous type antheridia are produced on regular-sized filaments, whereas, in nannandrous type antheridia are produced on special dwarf filaments called ‘dwarf male’ plants or nannandria (sing, nannandrium).
The macrandrous type may again he monoecious or dioecious. In macrandrous type antheridia are formed by successive transverse divisions of the vegetative cells of the filaments resulting in the formation of a row of antheridia (Fig. 65A). In every antheridium’ the contents divide into two, and each mass develops into an antherozoid also known as spermatozoid (Fig. 65B to E).
Except that they are much smaller and relatively poorer in chlorophyll content, the antherozoids resemble the zoospores, even to the characteristic crown of flagella (Fig. 65E).
The structure and distributional pattern of oogonia are identical in both macrandrous type and nannandrous type of species of Oedogonium. The oogonia are highly differentiated structures, each one originating from a vegetative cell behaving as oogonium mother cell (Fig. 66B).
This oogonium mother cell, by division forms two cells, the basal one becoming the supporting cell of the oogonium known as suffultory cell and the upper one the oogonium proper (Fig. 66A). The oogonium swells out, assuming a round or oval in outline (Fig. 66C).
The cell contents in the meantime withdraw themselves from the wall, and form a free, rounded protoplasmic body having a receptive spot—the ovum (Fig. 66D-E). Each oogonium bears one egg and opens, either by the formation of a pore or by the transverse splitting of the oogonial wall near the top corresponding to the receptive spot of the ovum, through which the swarming antherozoid reaches the immobile egg (Fig. 66E).
The male gamete penetrates the egg, and fertilization is effected when the nucleus of the antherozoid fuses with the egg nucleus, the resulting zygote is an oospore (Fig. 66F). The oospore develops a heavy wall and gradually passes into resting state (Fig. 66G).
After a period of rest, the oospore is set free either by the decay or rupturing of the oogonial wall (Fig. 66H). Meiosis precedes or accompanies germination of oospore with the formation of four haploid nuclei. The contents of the oospore divide into four parts each of which rounds itself off with a nucleus and becomes a zoospore (Fig. 66I & J).
These four zoospores resemble the zoospores formed in the vegetative cells, except that they are much smaller than the latter (Fig. 66K). The resulting zoospores develop into new filaments. In monoecious macrandrous type (e.g., Oedogonium fragile and O. nodulosum) the new filaments are of same kind (monoecious) (Fig. 67).
Whereas, in dioecious macrandrous type (e.g., Oedogonium crassum, two of the zoospores produce male filaments and the other two, female filaments (Fig. 68).
In nannandrous type, the sexual process is a little different from that of the macrandrous one, where an interesting dwarf male plant or nannandrium plays a very important role in the entire process. The dwarf male plants are developed from special type of zoospores known as androspores which are smaller than the usual zoospores and are associated with the sexual reproduction.
The androspores are prodded in androsporangia which are formed by repeated transverse divisions of the ordinary vegetative cells of the filament (Fig. 69A-). The contents of each of these androsporangia are transformed into a single zoospore (androspore) having a size intermediate between the usual zoospore and an antherozoid (Fig. 69B).
Most commonly, the androsporangia are produced from the same filament which bears the oogonia—the gynandrosporous (e.g., Oedogonium concatenatum); more rarely they occur on a separate filament—the idioandrosporous (e.g., Oedogonium confertum).
The androspore when liberated, after swarming, becomes attached either to the oogonial wall or to the wall of the suffultory cell, surrounds itself with a cell wall, and germinates to form one-celled dwarf male filament (Fig. 69G and I), having a rhizoid-like elongated attaching region or holdfast (Fig. 69C).
One or more antheridial mother cells are cut off from the tip of this dwarf male filament (Fig. 69D to F). Each antheridial mother cell develops into an antheridium which produces two antherozoids each bearing the characteristic crown of apical flagella. The antherozoids escape by a lid-like opening of the antheridium and make their way to the oogonial aperture (Fig. 70).
The act of fertilisation and formation of oospore is exactly the same as in macrandrous type. The entire process of formation of new haploid filaments from the oospore is also quite similar to the macrandrous one.
The development of androspores which is directly linked up with the sexual process in a nannandrous type is rather a very interesting aspect in the life cycle of this alga.
As to the origin of androspores, two hypotheses are considered:
(i) The androspores are equivalent to smaller zoospores that are produced from the oospores by reduction division and subsequent other processes which, in the nannandrous type, instead of producing normal filaments give rise to dwarf male individuals.
Whereas, in the macrandrous type there is no development of dwarf male filaments as the zoospores behave quite normally by producing normal filaments. As such, the macrandrous types are considered to be more advanced than the nannandrous ones.
(ii) The androspores are nothing but prematurely liberated antheridial mother cells which at later stage undergo further development and produce antherozoids. According to this view the macrandrous types are more primitive.
Some Indian species of Oedogoniaceae :
Oedogonium areolatum Lagarh.; O. armigerum Hirn.ƒ. tenuis Singh; O. cardiacum Kuta.; O. elegans Kutz.; O. oblongellum Kirch.; O. sociale Wittr.
Special features of Oedogoniaceae:
1. Filamentous un-branched plant body differentiated into base and apex.
2. Reticulate chloroplast.
3. Presence of caps in the dividing cells.
4. Very elaborate vegetative cell division.
5. Asexual reproduction by multi-flagellate zoospores having flagella arranged in a ring around a beak-like area—stephanokontan zoospores.
6. Sexual reproduction oogamous.
7. The female gamete is produced singly in each oogonium. It is retained in the oogonium until after fertilization.
8. The male gamete is very much similar to zoospore but much smaller in size.
9. A curious dimorphism of sexual plants is exhibited by the nannandrous type.
10. Formation of dwarf male—nannandrium by germination of multi-flagellate androspore.
11. The nannandrium produces antheridium which bears antherozoids.
12. Zygote produces four zoospores.
